One of the most challenging and formidable tasks in preparative organic chemistry is the selective functionalization of hydrocarbons. Once a functional group has been introduced, the chemist has a rich selection of tools to achieve further transformations and transpositions, but the initial barrier of introducing a functional group is determinative of further chemistry. Not only is it necessary that a given functionalization reaction proceeds in good yield, but it is necessary also that it proceeds with specificity. One of the most chemically attractive entry points to functionalization of hydrocarbons is the carbon-carbon double bond in alkanes and substituted alkenes, for the carbon-carbon double bond undergoes many reactions which introduce functional groups onto one or both of the carbons, and the double bond also activates an adjacent C-H bond (i.e., allylic hydrogen) to still other reactions. Among the chemical reactions of the carbon-carbon double bond that of epoxidation occupies a special niche, because epoxidation is virtually unique to the C.dbd.C linkage, because epoxidation proceeds with great specificity, and because the resulting epoxide is a controllably reactive functional group which reacts with a wide range of reagents, schematically represented here as H-Y, to afford an equally wide range of difunctional materials according to the reaction, ##STR1##
Although epoxidation may be performed with several different reagents, that variation of greatest interest here is one where the reagent is a hydroperoxide, particularly where epoxidation is catalyzed by a titanium compound. A commercial process uses tertiary butyl or ethylbenzene hydroperoxide in combination with 2% titania supported on silica to epoxidize propylene to propylene oxide with greater than 97% conversion of, for example, ethylbenzene hydroperoxide and selectivities to propylene oxide formation approaching 90%. See U.S. Pat. No. 3,642,833, 3,923,843, 4,021,454 and 4,367,342, all assigned to Shell Oil Company. More recently an Italian group has developed catalysts, referred to as titanium silicalites, where small amounts of framework silicon in silicalite are said to be replaced by titanium [Taramasso et al., U.S. Pat. No. 4,410,501] and has found such materials, conveniently designated as TS-1, to be effective in catalyzing the epoxidation of olefinic compounds by hydrogen peroxide in either the presence or absence of a solvent; U.S. Pat. No. 4,833,260. Subsequently this has been extended to the epoxidation of olefins with oxygen in air in the presence of a redox system of alkyl anthrahydroquinone and alkyl anthraquinone; EP 526,945.
Notaft, B., Innovation in Zeolite Materials Science; Grobet, P. J. et al., Ed.,; Elsevier: Amsterdam, pp. 422-424 has speculated that the observed catalytic activity both of titania supported on silica and TS-1 arises from the high dispersion of titanium atoms in a silica lattice, that is, active materials are characterized by Ti(IV) isolated by a long sequence of -O-Si-O-Si-. This conclusion was supported somewhat by the observation that when titania is supported on alumina, magnesia, or zirconia the resulting composite is inactive in epoxidation, and also is supported by the observation that catalyst activity increases as manifested by an increase in epoxide selectivity as the concentration of titania on silica decreases. Catalytic activity of TS-1 in the hydroxylation of phenol also has been shown to be dependent on particle size (A. J. H. P. van der Pol et al., Appt Catal., A92 (1992), 113-130) with particles in the 0.2-0.3 micron range being 10 times more active than those in the 5 micron range.
What we have observed is that particular mixtures of a titanosilicate and titania are demonstrably more active and more selective as a catalyst in the epoxidation of olefinic compounds than are prior art titanium-based catalysts which have been used in epoxidation. Whether the titanosilicate is analogous to the "titanium silicalite" exemplified by TS-1 may be open to dispute. What is not a matter of dispute is its greatly increased activity, selectivity, and reusability relative to TS-1. We believe that these characteristics make the catalysts of our invention uniquely suited for use in the epoxidation of olefins with hydrogen peroxide.